Also, when I do updates and OS upgrades using Desktop Manager, the list of languages I can add or remove never includes US English, so maybe it is just included by default and cannot be removed (which seems absurd).

I understand it seems absurd, but there's a very simple explanation to all of this. Language data that is worked on most of the time is US English, hence it'll always be more complete than other language data available for your BlackBerry. UK English and everything else comes second and in case you try accessing a portion of the software that has not been translated to the language option you have chosen, the system will fall back to US English.